When it is time to extend a well in a new direction, there were several options available. A whipstock could be used off an anchor to send a bit in a desired direction through a casing but there were some risks involved that the whipstock could shift and the exit could go off in an undesired location.
Section milling is a technique that removes a section of nested tubulars so that the well can be extended in a desired direction. In the past when there were nested tubulars the procedure was to cut each string with a separate mill with one mill being run into the well at a time. One of the fears that directed the multiple trip approach was that the initial cut by the smallest section mill would generate cuttings that could ball up the mill below if two mills were run at the same time. Another concern was that the two mill assembly if run into the hole at the same time would not be flexible enough as an assembly so that the second mill could center itself and avoid cutting another string outside the second string being cut in a situation where the strings were not concentric so that at some points the strings were literally in contact while in other peripheral locations at the same elevation there were large gaps between adjacent strings.
Most section cuts were made in a downhole direction. This technique increased the hole diameter for the circulating fluid as the cutting progressed which in turn required an increase in circulating fluid rate to maintain the velocity in the newly widened portion so that cuttings did not drop out. In one example the section milling was done in an uphole direction using a thruster to hold tension and an augur below a mill operated with either a downhole motor or string rotation to make the cut. This single mill method is illustrated in U.S. Pat. No. 6,679,328. Other related art to tubular section milling can be found in U.S. Pat. Nos. 6,227,313; 5,456,312; 5,373,900; 5,297,630; 5,150,755; 5,058,666; 4,978,260; 4,796,709; 4,938,291; 4,887,668; and 4,776,394. U.S. Pat. No. 5,265,675 shows a pivoting stabilizer combined with a single section mill.
What is needed and has not been available is a multiple mill system for section milling. The present invention offers such a system and incorporates into it an intermediate mill that can take what cuttings that travel to the adjacent mill to be made far smaller so that the second and larger mill will not get balled up when it is called upon to make the subsequent cut. The placement of an interim mill such as a watermelon mill between the pivoting arm type section mills gives the second mill a more flexible mounting so that it can center itself in the enlarged space created by the initial milling to lessen the possibility that an adjacent and off-center third string will not get cut as the second mill is operated. The second mill has features that keep its arms from extending until it gets out into the region cut by the initial mill. These and other features of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims.